Cementitious products

ABSTRACT

Macro defect free cementitious products are obtained by curing or firing a cementitious composition comprising water, a reactive filler and a water soluble alkali metal silicate. 
     The products show good strength and heat and water stability, by contrast with known mdf products where leaching out of the polymers used results in decreased strength.

This invention relates to cementitious products and more particularly toa method of making shaped and hardened cementitous products by curing orfiring compositions comprising reactive fillers and silicates. The term`cementitious products` is used herein to refer to products made fromcementitious compositions, i.e. compositions which are mouldable orotherwise shapable and which can be hardened, for example by curing orfiring.

The development of high strength cementitious products has been thefocus of much research activity. Methods which have been proposed formaking such products include the addition of fibrous and/or organicpolymeric materials to uncured cementitious compositions and thetreatment of conventional cementitious compositions to reduce the porevolume so as to give so-called macro-defect free cementitious products(see EP-A-0021682). A combination of these techniques has been describedin EP-A-0055035; however the macro-defect free (mdf) cementitiousproducts described in this latter specification suffer from the problemthat the polymer contained therein has a tendency to leach out of thecured cement on contact with water with consequent reduction of thestrength of the product. In addition the cured products in general havepoor heat stability even at temperatures as low as 80° C.

Various attempts have been made to solve the problems of poorwater-stability and poor thermal stability which have been encounteredwith mdf cementitious products. These have included treatment with boricacid (GB-A-2168292), burning out of the polymer followed by contact withwater (GB-A-2168293) and careful control of the nature of the calciumsilicate (GB-A-2162165). In addition it has been proposed to cross-linkthe polymers to hinder leaching out of the polymer (GB-A-2181726). Noneof these methods has however been entirely satisfactory.

We have now surprisingly found that compositions comprising one or morereactive fillers (e.g. pozzolanic materials) and one or more alkalimetal silicates can be used to make high strength mdf cementitiousproducts having both good water- and thermal stability. It has nothitherto been appreciated that the abovementioned compositions could besatisfactorily subjected to the high shear mixing processes required tomake mdf products and that they could subsequently be successfullyshaped and hardened, e.g. by forming into cohesive sheets and curing orfiring, to obtain mdf cementitious products having advantageousproperties.

According to one feature of the invention, therefore, there is provideda mdf cementitious product made by curing or firing of a cementitiouscomposition comprising water, at least one reactive filler (e.g. apozzolanic material) and at least one water soluble alkali metalsilicate, not more than 4% of the total volume of the product being madeup of pores of maximum dimension exceeding 100 microns. Preferably thesepores do not form more than 2%, more preferably not more than 0.5%, ofthe total volume of the product. The strength of the product is greaterthe smaller the proportion of relatively large pores and thus it ispreferred that not more than 2% of the total volume is made up of poresof maximum dimension exceeding 50 microns, more preferably 15 microns.It is also preferred that the total pore volume of the product does notexceed 20% of the total volume including pores, more preferably notexceeding 10% of the total volume.

According to a further feature of the invention, there is provided aprocess for the preparation of a cementitious product according to theinvention as hereinbefore defined which comprises milling a cementitiouscomposition as hereinbefore defined, by means of high shear mixing,shaping the milled composition thereby obtained and then hardening thecompositon by curing or firing.

The term "reactive filler" as used herein is intended to include fillermaterials capable of reacting chemically with an alkali metal silicateto form a cementitious composition or product.

Suitable reactive fillers are for example pozzolanic materials, the termpozzolanic being well known and defining a class of generally siliceousor aluminosiliceous materials which are not themselves cementitious butwhich react with slaked lime to yield a cementitious composition orproduct. Preferred examples of these materials suitable for use in theinvention include pulverised fuel ash (PFA) and blast furnace slag. Oneparticularly preferred reactive filler is a ground, granulated blastfurnace slag sold under the trade name CEMSAVE by Civil & Marine Ltd.,of West Thurrock, Grays, Essex, England. Other possible reactive fillersinclude underburnt clays and shales; burnt gaize; rice husk; bauxitewaste (a by-product of aluminium production); natural pozzolans such asvolcanic ashes; and silica fume. Silica fume is a known material,comprising extremely fine spherical silica particles; when fused to formhighly branched chain structures thixotropic silica fumes are obtained,while in non-thixotropic fumes there is no significant tendency to formbranched chains.

The reactive fillers for use in the invention preferably contain calciumas one of the cations present, for improved water stability. In view ofits lower calcium content, silica fume is advantageously used incombination with one or more calcium-containing reactive filers.

The reactive filler suitably consists of particles of less than 200microns diameter, preferably less than 100 microns.

The water-soluble alkali metal silicate used in the invention may forexample be a potassium or lithium or other silicate but sodium silicatesare preferred. Aqueous solutions of such silicates are highly basic andthe nature of the silicate anions is variable--they may, for example,exist as oligomeric polysilicates although shorter chains are alsopossible, including dimeric, trimeric and tetrameric silicates. Onesuitable material is sold under the Trade Mark KLEENODENSE 145K by AkzoChemie, Amersfoort, Netherlands, and is a viscous liquid of about pH 12containing oligomeric sodium and potassium silicates. This material issold for use in the production of Polysil concretes, these concretesconsisting generally of KLEENODENSE, fly ash and aggregate, but despiteits ready availability the possibility of using KLEENODENSE in theproduction of mdf cementitious products has not hitherto beenappreciated. Other suitable materials are Crosfield Crystal SodiumSilicate 100N, Crystal 112, Crystal 120A and Crystal 125, available fromCrosfield Chemicals Ltd, Warrington, England. In general thewater-soluble alkali metal silicates for use in the invention are lessexpensive than organic polymers which have previously been used in theproduction of high strength cementitious products.

It has been found that the higher viscosity silicates give particularlygood results in terms of strength of the resulting product. It istherefore preferred that the alkali metal silicate has a viscosity of atleast 500 centipoise, more preferably at least 1000 centipoise and stillmore preferably at least 2000 centipoise.

The precursor compositions used in the production of the mdfcementitious products according to the invention may optionally containadditional ingredients. For example, small quantities of hydrauliccement, for instance Portland cement or high-alumina cement, arepreferably included where it is desired to accelerate the settingreaction and improve the handling of the mix. Flow improving agents suchas glycerol may also preferably be included while it may also bedesirable to include a thickening agent where the alkali metal silicateis of lower viscosity. Other possible additives are for exampleunreactive fillers or aggregates such as fine silica sand, calciumcarbonate and talc.

Incorporation of fibres into the precursor compositions can give bettermilling properties and also better strength and impact resistance in thefinal product. Suitable fibres include nylon (polyamide), polypropylene,polyester and polyacrylonitrile fibres.

The proportions of the various ingredients are selected to provide aprecursor composition which can be satisfactorily mixed with high shearand subsequently shaped and hardened. In general the mix will preferablycontain 30 to 90%, more preferably 50 to 85% and still more preferably60 to 80%, by weight of reactive filler and 1 to 30% by weight, morepreferably 3 to 15% and still more preferably 5 to 12% by weight, ofalkali metal silicate (dry weight). As regards optional ingredients, theprecursor compositions will preferably contain up to 20%, morepreferably 2 to 20% and still more preferably 5 to 10%, by weight ofhydraulic cement, and up to 5%, more preferably 0.2 to 2%, of glycerol.As regards the water content of the precursor compositions, thepercentage of water in the mix is preferably 1 to 30%, more preferably 3to 20% and still more preferably 5 to 15%, by weight. All theabovementioned percentages are based on the total weight of the mix.

The precursor compositions are subjected to high shear mixing,preferably as hitherto described for use in the production of mdfcementitious products (see for example EP-A-0021682). This mixing may bein a bladed high shear mixer or a screw extruder but is most preferablyachieved by calendering in a twin roll mill by passing the compositionrepeatedly through the nip between the rolls of the mill with the rollsbeing rotated at different speeds, for instance in a ratio of about 4:5or about 10:11.

It will be appreciated that the consistency of the mix is important indetermining the milling properties and that the amount of water andother ingredients should be selected to provide a millable composition.The flow behaviour of the millable precursor compositions is desirablysuch that they readily flow under minimum force as described inEP-A-55035 and in Phil. Trans. R. Soc. Lond. A310, pages 139-153 (1983),i.e. they possess the so-called "Bingham" type flow behaviour. Such flowbehaviour results in good milling properties and high strength in theresulting cured or fired product. The rolls are preferably set toprovide a calendered sheet having a thickness of not more than 20 mm,e.g. of about 1 or 2 to 20 mm, and the calendering is repeated until asubstantially homogeneous calendered sheet is obtained.

Following thorough mixing the compositions are shaped and then hardenedby curing or firing to provide the mdf cementitious products accordingto the invention. The shaping step conveniently comprises pressing,again as hitherto described for the production of mdf cementitiousproducts, for instance at a pressure in the range 0.5 to 5 MPa,preferably 2 to 4 MPa. It may then be convenient to subject the shapedproduct to an initial setting step to provide shape stability prior tocuring or firing. Setting can be carried out at ambient or elevatedtemperatures, e.g. at a temperature of from 10° C. to 100° C. for aperiod of 5 minutes to 24 hours. Where shaping is effected by pressingthe setting step can be carried out with the pressure still applied.Following setting, the shaped product can if desired be moved, e.g. to acuring oven. Curing is preferably effected at temperatures within therange of from 10° to 120° C., more preferably 60° to 100° C., forperiods of from 6 hours to 7 days or more, for example overnight. Curingis advantageously effected under humid conditions, for instance with theproducts enclosed in plastic bags or other containers to avoid shrinkagedue to water loss. Much faster curing can if desired be achieved by theuse of microwave radiation whereby curing can for example be completedwithin less than one hour. As an alternative to curing, the compositionsmay be fired e.g. at temperatures of from 600° to 1200° C., preferably800° to 1000° C., for periods of from 15 minutes to 2 hours, preferably30 minutes to 1.5 hours.

The above described processes for preparation of mdf cementitiousproducts constitute further features of the present invention.

The production of mdf cementitious products of high flexural strength,with for example MOR values in excess of 15 MPa, makes possible the useof this novel material for a wide range of applications, for exampleroof tiles and roofing slates of high flexural strength. Other possibleuses for the mdf cementitious products according to the invention are asmaterials for domestic and industrial flooring tiles, wall tiles,panelling for walls, ceilings, floors and roofing, corrugated sheeting,e.g. for fencing and roofing, and extruded piping.

The invention is illustrated by the following Examples. Pulverised fuelash was obtained under the trade name Pozzolan from Pozzolanic-LytagLtd., Hemel Hempstead, Herts, England; Kleenodense 145K from AkzoChemie, Amersfoort, Netherlands; ciment fondu was obtained from LafargeAluminous Cement Co. Ltd., Grays, Essex, England. Crosfield CrystalSilicates were obtained from Crosfield Chemicals Ltd, Warrington,England; Omya BLR3 chalk was obtained from Croxton & Garry Ltd, ofDorking, Surrey, England; Elkem Micro Silica from Elkem ChemicalsLimited of High Wycombe, Bucks., England.

EXAMPLE 1

A mix was prepared containing 74.5% by weight of pulverised fuel ash(Pozzolan), 21% by weight of Kleenodense 145K (containing 46.8% solids,53.2% water), 4% ciment fondu (high alumina cement) and 0.5% glycerol.Mixing took place in a Hobart planetary mixer for 1 to 2 minutes; themix was then transferred to a twin roll mill with roll speeds in theratio of 4:5 and mixed by passing approximately five times between therolls to form a homogeneous sheet of thickness approximately 3 mm. Thecalendered material was then transferred to a press with a flat mouldfor pressing at 3 MPa for 10 minutes at ambient temperature.

Following pressing the sheet was subjected to further setting in an ovenat 80° C. for 30 minutes and then demoulded and cured at 80° C.overnight in a sealed bag.

Test pieces (80×40×3 mm) were prepared from the cured sheet.

The three-point modulus of rupture values were determined using thefollowing formula: ##EQU1## where M.O.R.=Modulus of Rupture (MPa)

W=Breaking load (N)

L=Span between supports (mm)

d=Thickness (mm)

b=Width (mm)

The following result was obtained:

    Dry strength=12.4 MPa.

EXAMPLE 2

A formulation was prepared, milled and cured to set the material forease of handling as in Example 1.

Once cured the sheet was fired at 1000° C. for 1 hour, after which itwas cooled to room temperature.

Test pieces (80×40×3 mm) were prepared from the fired sheet.

The three-point modulus of rupture value was determined as 25.8 MPa(dry).

EXAMPLE 3-6

Macro defect free materials were prepared to the formulations describedin TABLE 1. Mixing and material processing were as described in Example1.

Test pieces (80×40×3 mm) were prepared from the cured sheet.

The three-point modulus of rupture values were determined and are alsoshown in TABLE 1.

                                      TABLE 1                                     __________________________________________________________________________                  Example                                                                       3        4        5        6                                                  %        %        %        %                                    __________________________________________________________________________    POZZOLAN      69.2     65.3     65.3     65.3                                 (Pulverised Fuel Ash)                                                         ORDINARY PORTLAND                                                                            8.7      8.5      8.5      8.5                                 CEMENT                                                                        KLEENODENSE 145K                                                                            22.1     --       --       --                                   CRYSTAL 112   --       26.2     --       --                                   CRYSTAL 120A  --       --       26.2     --                                   CRYSTAL 125   --       --       --       26.2                                 MODULUS OF RUPTURE                                                                          12.0     13.1     14.2     20.3                                 (Dry [MPa])                                                                   MODULUS OF RUPTURE                                                                           6.9     15.5     10.9     14.4                                 (Wet [MPa])                                                                   PRESSING AND  3 MPa, RT*,                                                                            3 MPa, RT,                                                                             3 MPa, RT,                                                                             3 MPa, RT,                           CURING DETAILS                                                                              20-30 min.;                                                                            30 min.; 30 min.; 30 min.;                                           14 days at 50° C.                                                               24 hrs. at 50° C.                                                               264 hrs. at 60° C.                                                              264 hrs. at 60° C.                          enclosed in a                                                                          enclosed in a                                                                          enclosed in a                                                                          enclosed in a                                      plastic bag                                                                            plastic bag                                                                            plastic bag                                                                            plastic bag                          __________________________________________________________________________     *RT = Room Temperature.                                                  

EXAMPLES 7-10

Macro defect free materials were prepared to the formulations describedin TABLE 2. Mixing and material processing were as described in Example1.

Test pieces (80×40×3 mm) were prepared from the cured sheets.

The three-point modulus of rupture values were determined and are alsoin TABLE 2

In order to test the resistance of products to warping, sheets ofexamples 7 and 8 were placed with one face in contact with water for twoweeks. No warpage was found after this period. Examples 7 and 8 werealso submitted to pore size analysis by mercury intrusion porosimetry.The diameter of the maximum pore size in both cases was found to be 0.4microns.

                                      TABLE 2                                     __________________________________________________________________________                  Example                                                                       7        8        9        10                                                 %        %        %        %                                    __________________________________________________________________________    BLAST FURNACE SLAG                                                                          75.8     72.5     72.5     72.5                                 (Cemsave)                                                                     KLEENODENSE 145K                                                                            24.2     --       --       --                                   CRYSTAL 112   --       27.5     --       --                                   CRYSTAL 120A  --       --       27.5     --                                   CRYSTAL 125   --       --       --       27.5                                 MODULUS OF RUPTURE                                                                          34.9     35.4     55.3     48.6                                 (Dry [MPa])                                                                   MODULUS OF RUPTURE                                                                          28.1     29.9     39.2     41.6                                 (Wet [MPa])                                                                   PRESSING AND  3 MPa, RT,                                                                             3 MPa, RT,                                                                             1.5 MPa, RT,                                                                           1.5 MPa, RT,                         CURING DETAILS                                                                              15 min.; 15 min.; 30 min.; 30 min.;                                           24 hrs. at 50° C.                                                               24 hrs. at 50° C.                                                               120 hrs. at 80° C.                                                              120 hrs. at 80° C.                          144 hrs. at 80° C.                                                              144 hrs. at 80° C.                                                              enclosed in a                                                                          enclosed in a                                      enclosed in a                                                                          enclosed in a                                                                          plastic bag                                                                            plastic bag                                        plastic bag                                                                            plastic bag                                            __________________________________________________________________________

EXAMPLES 11 and 12

The addition to the basic formulation of a fine reactive filler, namelysilica fume, was investigated.

The formulations in TABLE 3 were made up in accordance with the mixingand processing instructions described in Example 1.

Test pieces (80×40×3 mm) were prepared from the cured sheets.

The three-point modulus of rupture values were determined and are alsogive in TABLE 3.

                  TABLE 3                                                         ______________________________________                                                       Example                                                                       11        12                                                                  %         %                                                    ______________________________________                                        PULVERISED FUEL ASH                                                                            67.8        --                                               (Pozzolan)                                                                    BLAST FURNACE SLAG                                                                             --          74.6                                             (Cemsave)                                                                     ORDINARY PORTLAND                                                                               9.2        --                                               CEMENT                                                                        KLEENODENSE 145K 16.3        17.9                                             SILICA FUME       6.8         7.5                                             (Elkem Micro Silica)                                                          MODULUS OF RUPTURE                                                                             22.2        40.4                                             (Dry [MPa])                                                                   MODULUS OF RUPTURE                                                                              21.75      46.3                                             (Wet [MPa])                                                                   PRESSING AND     3 MPa, RT,  3 MPa, RT,                                       CURING DETAILS   15 min.;    15 min.;                                                          24 hrs. at  24 hrs. at                                                        50° C.                                                                             50° C.                                                     144 hrs. at 144 hrs. at                                                       80° C.                                                                             80° C.                                                     enclosed in a                                                                             enclosed in a                                                     plastic bag plastic bag                                      ______________________________________                                    

EXAMPLES 13 and 14

The addition to the basic formulation of a fine inert filler, namelyOmya BLR3 Chalk, was investigated.

The formulations in TABLE 4 were made up in accordance with the mixingand processing regimes described in Example 1.

Test pieces (80×40×3 mm) were prepared from the cured sheet.

The three-point modulus of rupture values were determined and are alsoshown in TABLE 4.

                  TABLE 4                                                         ______________________________________                                                       Example                                                                       13        14                                                                  %         %                                                    ______________________________________                                        PULVERISED FUEL ASH                                                                            63.1        --                                               (Pozzolan)                                                                    BLAST FURNACE SLAG                                                                             --          69.0                                             (Cemsave)                                                                     ORDINARY PORTLAND                                                                              8.5         --                                               CEMENT                                                                        KLEENODENSE 145K 22.1        24.1                                             OMYA BLR3 CHALK  6.3         6.9                                              MODULUS OF RUPTURE                                                                             11.76       29.08                                            (Dry [MPa])                                                                   MODULUS OF RUPTURE                                                                             10.11       36.15                                            (Wet [MPa])                                                                   PRESSING AND     3 MPa, RT,  3 MPa, RT,                                       CURING DETAILS   15 min.;    15 min.;                                                          24 hrs. at  24 hrs. at                                                        50° C.                                                                             50° C.                                                     144 hrs. at 144 hrs. at                                                       80° C.                                                                             80° C.                                                     enclosed in a                                                                             enclosed in a                                                     plastic bag plastic bag                                      ______________________________________                                    

EXAMPLE 15

The formulation in TABLE 5 was prepared, milled and cured to set thematerial for ease of handling as described in Example 1.

Once cured the resultant sheet was fired at 1000° C. for 1 hour, afterwhich it was cooled to room temperature.

Test pieces (80×40×3 mm) were prepared from the fired sheet.

The three-point modulus of rupture values were determined and are alsogiven in TABLE 5.

Solids contents of alkali metal silicates

The solids contents of the alkali metal silicates used in the aboveExamples are given below:

    ______________________________________                                        Silicate          Solids Content                                              ______________________________________                                        KLEENODENSE 145K  46.8%                                                       CRYSTAL 100N      43.6%                                                       CRYSTAL 112       45.8%                                                       CRYSTAL 120A      48.2%                                                       CRYSTAL 125       49.7%                                                       ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                       Example                                                                       16                                                                            %                                                              ______________________________________                                        BLAST FURNACE SLAG                                                                             72.5                                                         (Cemsave)                                                                     CRYSTAL 100N     27.5                                                         MODULUS OF RUPTURE                                                                             44                                                           (Dry [MPa])                                                                   MODULUS OF RUPTURE                                                                             39                                                           (Wet [MPa])                                                                   PRESSING AND     3 MPa, RT, 10 min.; 24 hrs.                                  CURING DETAILS   at 50° C. enclosed in a plastic                       ______________________________________                                                         bag                                                      

We claim:
 1. A cementitious product comprising the reaction product of1%-30% by weight of water; 30%-90% by weight of reactive filler; and1%-30% by weight of water soluble alkali metal silicate; said reactivefiller being a siliceous or aluminosiliceous material capable ofreacting with said alkali metal silicate to form a cementitiouscomposition; wherein the total volume of the product comprises not morethan 4% of pores having a maximum dimension exceeding 100 microns; theproduct having a MOR value of at least 15 MPa.
 2. A cementitious productaccording to claim 1, wherein the total volume of the product comprisesnot more than 2% of pores having a maximum dimension exceeding 100microns.
 3. A cementitious product according to claim 1, wherein thetotal volume of the product comprises not more than 2% of pores having amaximum dimension exceeding 50 microns.
 4. A cementitious product asclaimed in claim 1, wherein the total volume of the pores in the productdoes not exceed 20% of the total volume of the product.
 5. Acementitious product as claimed in claim 1, wherein the cementitiouscomposition contains a pozzolanic reactive filler.
 6. A cementitiousproduct as claimed in claim 5, wherein the pozzolanic reactive filler ispulverized fuel ash or blast furnace slag.
 7. A cementitious product asclaimed in claim 1, wherein the water soluble alkali metal silicate issodium silicate.
 8. A cementitious product as claimed in claim 5,wherein the water soluble alkali metal silicate is sodium silicate.
 9. Acementitious product as claimed in claim 1, wherein the cementitiouscomposition further comprises from 0 to 20% by weight of hydrauliccement, and from 0 to 5% by weight of glycerol.
 10. A cementitiousproduct as claimed in claim 8, wherein the cementitious compositionfurther comprises from 0 to 20% by weight of hydraulic cement, and from0 to 5% by weight of glycerol.
 11. A cementitious product as claimed inclaim 1 in the form of a roof tile or roofing slate.
 12. A cementitiousproduct as claimed in claim 8 in the form of a roof tile or roofingslate.
 13. A cementitious product as claimed in claim 9 in the form of aroof tile or roofing slate.
 14. A cementitious product as claimed inclaim 10 in the form of a roof tile or roofing slate.
 15. A cementitiousproduct as claimed in claim 1, wherein the alkali metal silicate has aviscosity of at least 500 centipoise.
 16. A cementitious product asclaimed in claim 1, wherein the reactive filler comprises calcium andsilica fume.
 17. A process for the preparation of a cementitiousproduct, wherein a cementitious composition comprising(i) from 1 to 30%by weight of water; (ii) from 30 to 90% by weight of at least onereactive filler, which is capable of reacting chemically with an alkalimetal silicate to form a cementitious composition; and (iii) from 1 to30% by dry weight of at least one water soluble alkali metal silicate;ismilled by means of high shear mixing, shaped, and then hardened bycuring or firing, so that the total volume of the product is comprisedof not more than 4% of pores having a maximum dimension exceeding 100microns.
 18. A process as claimed in claim 17 wherein the cementitiouscomposition is subjected to high shear mixing by calendering in a twinroll mill.